Upstream needs this to deal with size_t, but our low-level DES APIs take
size_t, so this is not a concern.

Change-Id: I9dc4c7248c5dd9515246a4b224147b932328a400
Reviewed-on: https://boringssl-review.googlesource.com/3882
Reviewed-by: Adam Langley <agl@google.com>

Initial fork from f2d678e6e8 (1.0.2 beta).

(This change contains substantial changes from the original and
effectively starts a new history.)